This invention relates generally to sensor networks such as used for example in traffic control, and more particularly the invention is directed to a power efficient system for communicating in sensor networks.
Wireless sensor networks is an emerging research area with potential applications in environmental monitoring, surveillance, military, health and security. Such a network consists of a group of nodes, called sensor nodes, each with one or more sensors, an embedded processor, and a low power radio. Typically, these nodes are linked by a wireless medium to perform distributed sensing tasks.
Many wireless sensor network applications require power efficiency, real-time guarantee, congestion control and fairness simultaneously. Examples of these are parking lot, traffic light, factory monitoring, fire detection, and security monitoring applications.
Wireless sensor networks can be used in parking lots to determine free spots and relay this information to a central data base or Access Point (AP) as shown in FIG. 1. Each parking spot contains one or more sensor nodes. These sensor nodes detect the presence of the car in their spot by using magnetic or acoustic sensor and then relay this information to AP. The AP provides information about vacant parking spaces. This application can save customers time and increase revenues with higher occupancy rates by eliminating turnaround areas and quick detection of vacant spaces. The primary requirement in this application is real-time delivery guarantee of packets so that the AP has up-to-date information about free spaces. Moreover, power efficiency is essential to decrease the maintenance cost of the system. Congestion control and fairness are important to get at least one packet from each parking space within a specific amount time.
A sensor network application at a traffic light involves adjusting the cycle time of traffic light based on the density of the cars behind lights, determined by using the sensor nodes as shown in FIG. 2. The number of cars passing through each node is calculated with the aid of a magnetic or acoustic sensor in the node. This information is periodally sent to the traffic light controller so that the controller can figure out the number of cars in each area and adjust the cycle time in the next round appropriately. This application requires real-time delivery guarantee with 30-60 sec, in order that the decision is optimal for the next round. Power efficiency is important to decrease the cost of the system. Congestion control and fairness are necessary to get the traffic information from each area of the road.
Machine diagnosis in an industrial setting is another use of a sensor network, in which energy constrained sensor nodes communicate to a single high-powered base station. This application again requires strict guarantee on the latency. Power efficiency and fairness decrease the maintenance cost of the system and provide information about each machine.
Fire detection and security monitoring are other applications of sensor networks with strict requirements. After deploying the sensor nodes in an area, the sensor network should be able to guarantee that the fire or security breach is detected and notified within a specific amount of time. Moreover, the company deploying this network should give a lower bound of at least 3-4 years on the lifetime of the system. Fairness should be satisfied in order to monitor every part of the area.
The basic feature of a sensor network that is different from traditional wireless ad hoc networks is that data traffic flow is from the sensor nodes to an access point (AP) that collects the data, rather than many independent point-to-point flows. Another important sensor network characteristic is that traffic generation at each node either has to be periodic or can be made periodic for robustness of the system. For instance, monitoring each spot in parking lot in order to lead the cars to empty spaces may require periodic packet generation at each sensor node. On the other hand, the sensor network deployed for fire detection needs packet generation only when there is a fire. However, if the network is not functional due to node failures, the AP will interpret this as having no fire. The periodic update of the fire condition by periodic generation of packets in the sensor nodes justifies the robust operation of the system.
The energy limitation of the sensor nodes due to their small size and long lifetime requirements imposes constraints on the protocol design. The primary source of energy consumption is the radio. Collision causes a packet to be corrupted by another packet. Since this packet is discarded, the energy consumption per successful transmission will increase. Idle listening occurs when the node consumes power listening to the channel for possible traffic even when there is no packet to be received on the channel. Overhearing occurs when a node consumes energy to receive a packet that is not destined to itself. Finally, control packets should be minimized to eliminate the energy consumption related to them. Since listening to the channel or receiving a packet may cost almost as much power as transmitting a packet, sensor nodes must only be awake to receive the packets destined to themselves or to transmit, and sleep otherwise in order to conserve power.